The culprit, they say, is a new pathway that underlies sensitivity to light
during migraine episodes in blind people and people with normal eyesight.
Scientists at Beth Deaconess Medical Center report their findings in the online
journal Nature Neuroscience.

Migraines, which afflict more than 30 million people in the United States,
are painful headaches for which sufferers often seek relief by going into dark
rooms. Migraines are believed to develop when the system of membranes
surrounding the brain and central nervous system, called the meninges, becomes
irritated.

That stimulates pain receptors and triggers a series of events that lead to
prolonged activation of groups of sensory neurons, the scientists report.

Migraines cause one-sided, throbbing pain, and have been called “sick
headaches” because they have long been associated with vomiting, nausea,
irritability, and fatigue.

People with migraines also are also typically aggravated by light with
worsening of symptoms compared to being in the dark. This causes many migraine
sufferers to wear sunglasses, often at night, Burstein says.

It was the observation that even blind people who suffer migraines were
experiencing sensitivity to light, referred to as photophobia, that led
Burstein and Rodrigo Noseda, PhD, to hypothesize that signals transmitted from
the retina to the optic nerve were triggering the intense headaches, the
authors say.

The scientists studied two groups of blind people with migraines. Patients
in one group were totally blind because of eye diseases, unable to see images
or sense light.

People in a second group were legally blind, but were able to detect the
presence of light.

“While the patients in the first group did not experience any worsening of
their headaches from light exposure, the patients in the second group clearly
described intensified pain when they were exposed to light, in particular blue
or gray wavelengths,” Burstein says in a news release. “This suggested to us
that the mechanisms of photophobia must involve the optic nerve, because in
totally blind individuals, the optic nerve does not carry light signals to the
brain.”

He says they suspected that a group of recently discovered retinal cells
that help control biological functions, including sleep and wakefulness, were
critically involved in this process, because “these are the only functioning
light receptors left among patients who are legally blind,” Burstein says.

In laboratory experiments involving animals, they traced the path of
specific cells through the optic nerve to the brain. A group of neurons then
became electrically active.

“When small electrodes were inserted into these ‘migraine neurons’, we
discovered that light was triggering a flow of electrical signals that was
converging on these very cells,” Burstein says. “This increased their activity
within seconds.”

When the light was taken away, the neurons remained activated, which
explains “why patients say that their headache intensifies within seconds after
exposure to light, and improves 20 to 30 minutes after being in the dark,”
Burstein says.

The discovery of the new pathway will provide scientists a new path to
follow in trying to solve photophobia, he says.

The study, the authors write, reveals “a mechanism for the exacerbation of
migraine headache by light.”

“Clinically, this research sets the stage for identifying ways to block the
pathway so that migraine patients can endure light without pain,” Burstein
says.